A considerable body of experimental evidence has accumulated which suggests that atrial receptors might play an important physiological role in the regulation of heart rate and body fluid volume. However, the distinct receptor types involved in these reflexes and their functional properties have not yet been established. An in vitro preparation using the rat atrium and vagus nerve has been developed to answer these questions. Single unit or few fiber afferent responses will be recorded. Computer programs have been developed to analyze quantitatively the steady-state and dynamic characteristics of type A and B atrial receptors and of atrial receptors with unmyelinated fibers. The contribution of the excitable properties of the receptor membrane and axon to the adaptation of the neural discharge will be assessed. Various patterns of electrical and mechanical stimuli (i.e., square-wave steps, ramps, sine waves and random noise) will be tested. The effects on the stimulus-response curves will also be studied by using pharmacological agents and by altering the ionic environment of the receptor.